Having a rails application on the vagrant machine and the 3000 port not being forwarded when you access it form host machine like

http://localhost:3000

if so keep reading

Assuming you have already setup your vagrant machine

Go ahead and open your Vagrant file, find the config.vm.network and update it with

config.vm.network "forwarded_port", guest: 3000, host: 3000

Then make sure you are binding the server with 0.0.0.0 as it is the main problem is being the default 127.0.0.1 for that, when yoiu are instantiation the server do the binding with 0.0.0.0

bin/server --binding=0.0.0.0

Basically the above would tell to listen to almost any ip. This has its own risk if you are doing it on public VM so caution shall be applied. Where as when it is 127.0.0.1, it will be listening to the local address only.